(1) Field of the Invention
This invention relates to alternative phase shifting masks for patterning a layer of negative photoresist to form a contact hole mask and to a method of using the alternative phase shifting masks to pattern a layer of negative photoresist to form a contact hole mask.
(2) Description of the Related Art
Conventional binary masks have been used to form contact hole masks in a layer of photoresist. As the contact holes become smaller phase shifting masks have been used to pattern the photoresist.
U.S. Pat. No. 5,498,579 to Borodovsky describes multiple exposures of a photosensitive layer to improve resolution in the patterning of the photosensitive layer.
U.S. Pat. No. 5,702,848 to Spence describes the use of phase shift masks using step regions that compensate for 0xc2x0 to 180xc2x0 transition effects.
U.S. Pat. No. 5,798,203 to Haraguchi et al. describes a method of forming a photoresist image using two exposures of the photoresist. The resist used is a positive resist and is exposed. The exposed resist is then changed in its character by baking the substrate in an amine gas to make an exposed portion insoluble in developer. An unexposed portion of the positive resist is then exposed a second time.
In order to form electrical connections between wiring layers separated by a layer of interlevel dielectric or to form electrical connections to integrated circuits it is necessary to form contact holes or via holes through layers of dielectric material. As the dimensions of these contact holes or via holes becomes smaller the conventional binary mask no longer produces contact holes or via holes of adequate quality. In these situations phase shifting masks are frequently used. Conventional phase shifting masks used for formation of contact holes or via holes require masks having layers of opaque as well as phase shifting material and are complex masks compared to the binary masks. Defects in the part of the mask covered by opaque material will cause problems when the masks are used to form contact holes or via holes.
FIGS. 1 and 2 show a conventional binary mask used to expose a layer of photoresist in order to fabricate contact holes in an integrated circuit wafer. FIG. 1 shows the top view of a conventional binary mask 20A used for contact holes showing the contact hole regions 22A formed in the layer of opaque material 24 exposing the transparent mask substrate 26 beneath the layer of opaque material. FIG. 2 shows a cross section of the conventional binary mask 20A taken along line 2-2xe2x80x2 of FIG. 1. FIG. 2 shows the layer of opaque material 24 formed on the transparent mask substrate 26 with the contact hole regions 22A formed in the layer of opaque material 24.
FIGS. 3-6 show a set of two conventional phase shifting masks, 20B and 20C, used to expose a layer of photoresist in order to fabricate contact holes in an integrated circuit wafer. FIG. 3 shows a top view and FIG. 5 a cross section view of the first mask 20B. The first mask 20B has a layer of opaque material 24 formed on a transparent mask substrate with contact hole regions 22B formed in the layer of opaque material 24. Each of the contact hole regions 22B has a first region 28 and a second region 29. Part of the transparent mask substrate 26 is removed from the second region so that the transparent mask substrate material remaining in the first region 28 provides a 180xc2x0 phase shift to light passing through the first region 28 relative to light passing through the transparent mask substrate material remaining in the second region 29. FIG. 4 shows a top view and FIG. 6 a cross section view of the second mask 20C. The second mask 20C has a layer of opaque material 24 formed on a transparent mask substrate with contact hole regions 22C formed in the layer of opaque material 24. Each of the contact hole regions 22C has a first region 28 and a second region 29. Part of the transparent mask substrate 26 is removed from the second region so that the transparent mask substrate material remaining in the first region 28 provides a 180xc2x0 phase shift to light passing through the first region 28 relative to light passing through the transparent mask substrate material remaining in the second region 29. As can be seen in FIGS. 3 and 4 the region of the first mask providing a 180xc2x0 phase shift is rotated 90xc2x0 with respect to the region of the second mask providing a 180xc2x0 phase shift.
FIG. 5 shows a cross section of the conventional first phase shift mask 20B taken along line 5-5xe2x80x2 of FIG. 3. FIG. 5 shows the layer of opaque material 24 formed on the transparent mask substrate 26 of the conventional first phase shift mask 20B with the contact hole regions 22B formed in the layer of opaque material 24 of the first mask 20B and part of the transparent mask substrate removed from the second region 29 of the first mask 20B. FIG. 6 shows a cross section of the conventional second phase shift mask 20C taken along line 6-6xe2x80x2 of FIG. 4. FIG. 6 shows the layer of opaque material 24 formed on the transparent mask substrate 26 of the second mask 20C with the contact hole regions 22C formed in the layer of opaque material 24 of the second mask 20C and part of the transparent mask substrate 26 removed from the second region 29 of the second mask 20C
The conventional first phase shifting mask and the conventional second phase shifting mask are used sequentially to expose a layer of photoresist. The conventional phase shifting masks require a layer of opaque material to form the masks. Defects in the opaque material will adversely affect the photoresist pattern formed using the masks.
It is a principle objective of this invention to provide a phase shifting mask to be used with negative photoresist for the formation of contact holes or via holes which does not require the use of opaque material and is tolerant of mask defects.
It is another principle objective of this invention to provide a method of forming contact hole or via hole patterns in a layer of negative photoresist using phase shifting masks having no opaque material which are tolerant of defects.
These objectives are achieved with two phase shifting masks having no opaque material. These masks are used to expose a layer of negative photoresist. A first phase shifting mask provides a 90xc2x0 phase shift to light passing through a first half of each of the contact hole regions of the first mask and a 270xc2x0 phase shift to light passing through the second half of each of the contact hole regions of the first mask. These phase shifts are relative to the remainder of the first mask. A second phase shifting mask provides a 90xc2x0 phase shift to light passing through a first half of each of the contact hole regions of the second mask and a 270xc2x0 phase shift to light passing through the second half of each of the contact hole regions of the second mask. These phase shifts are relative to the remainder of the second mask. The first half of each of the contact hole regions of the first mask are rotated 90xc2x0 with respect to the first half of each of the contact hole regions of the second mask. The second half of each of the contact hole regions of the first mask are rotated 90xc2x0 with respect to the second half of each of the contact hole regions of the second mask.
A layer of negative photoresist is then exposed by the first mask and by the second mask. The layer of negative photoresist is then developed which removes the unexposed portions of the negative photoresist. The interference patterns at the boundaries of regions of different phase shift form unexposed regions in the contact hole regions of the mask. Since the masks are fabricated entirely from transparent material, defects in the mask in mask regions other than the contact hole regions of the mask are not a concern.